A classic interview question covering 4 hot topics

1.ConcurrentHashMap Use synchronized Keywords ensure thread safety

2.HashMap Realized Collction Interface

3.Array.asList Method returns java.util.ArrayList object

4.SimpleDateFormat It's not thread safe

A. JDK1.8 Of ConcurrentHashMap use CAS+Synchronized Ensure thread safety . JDK1.7 And previously used segment Thread security is realized by the segmented lock mechanism of , among segment Inherited from ReentrantLock, So we use Lock Lock to ensure thread safety .

B. 

C. Arrays.asList()   return  java.util.Arrays.ArrayList object , there ArrayList yes Arrays Private inner class

D.